This R01 application is in response to the recent RFA, the Impact of Child Psychopathology & Childhood Interventions on Subsequent Drug Abuse, and is to study the relationship between Post-traumatic Stress Disorder (PTSD) and substance use/abuse in youth. With the 9/11 terrorist attack on the World Trade Center, subsequent terrorist threats and documented elevated rates of PTSD in youth, it is important to understand the relationship between PTSD and subsequent substance use/abuse, if we are to provide adequate prevention and intervention to this population, particularly in light of possible future terrorist attacks. The proposed study is based on secondary data analysis of 4 existing data sets with information about PTSD and substance use/abuse in youth. Two of the data sets are from two New York City Board of Education School Surveys conducted at 6 months (grades 6 - 12, N=4,690) and 18 months (grades 6-12, projected N=4,000) after 9/11. Two others are longitudinal data sets from community samples. The Children in the Community (CIC, N=717) sample, with children randomly selected from two upstate New York counties in 1975 at mean age of 5, followed-up 5 times, including as young adults. Also, a sub-sample from the Boricua Youth Study, those ages who were 10 to 15 at the baseline interview (N=1,300), and re-interviewed twice at 12 month intervals. This study will examine: (1) whether or not PTSD (or PTSD symptoms) after 9/11 are associated with increased substance use and abuse in youth; (2) whether or not youth use substances to self-medicate their PTSD symptoms after 9/11; (3) whether or not youth exposure to trauma and related PTSD (or symptoms) has an impact on later substance use and abuse; (4) whether or not the relationship between PTSD (or symptoms) and substance use/abuse differ by socio-demographic factors, such as gender, ethnicity, and family SES; and (5) whether or not the relationship between PTSD (or symptoms) and substance use/abuse can be explained by the existence of other comobid psychiatric disorders, such as major depression and anxiety. It is hoped that the findings from this project will provide useful information for clinicians and policy makers, so as to improve service delivery for youth with PTSD and substance use/abuse after traumatic events.